1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED), and more particularly, to an active matrix type organic light emitting diode (AMOLED) device and a thin film transistor thereof.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been most widely used in the field for flat panel display devices due to their lightweight and low power consumption. However, the liquid crystal display (LCD) device is not a light emitting element but a light receiving element that needs an additional light source to display images. Thus, there have existed technical limitations in improving brightness, contrast ratio, viewing angle, and enlarging the size of a liquid crystal display panel. For these reasons, many researches have been in active progress in the field to develop a new flat panel display element that can overcome the aforementioned problems. The organic light emitting diode (OLED) device is one of the new flat panel display elements. Because the organic light emitting diode (OLED) device emits light without an additional light source, the viewing angle and the contrast ratio are superior to the liquid crystal display (LCD) device. In addition, because it does not need a backlight as the light source, it has advantages such as lightweight, small dimension, and low power consumption. Moreover, the organic light emitting diode (OLED) device can be driven with a low DC (direct current) and has a fast response time. Since the organic light emitting diode (OLED) device uses solid material instead of fluid material, such as liquid crystal, it is more stable under the external impact and has a wider range of temperature conditions under which the organic light emitting diode (OLED) device can be operated, as compared to the liquid crystal display (LCD) device. Specifically, the organic light emitting diode (OLED) device is advantageous in that the production cost is low. Specifically, because a deposition apparatus and an encapsulation apparatus are the only apparatuses required for manufacturing the organic light emitting diode (OLED) device, as opposed to the liquid crystal display (LCD) device or plasma display panels (PDPS) needing many other apparatuses, the manufacturing process for the organic light emitting diode (OLED) device becomes very simple. If the organic light emitting diode (OLED) device is an active matrix type organic light emitting diode (AMOLED) device that has a thin film transistor as a switching element for each pixel, a desired luminance can be obtained by applying a low current. Accordingly, the organic light emitting diode (OLED) device is advantageous in terms of a low power consumption, a high resolution, and a large size. The basic structure and operation characteristics of the active matrix type organic light emitting diode (AMOLED) device will be described hereinafter with reference to FIG. 1.
FIG. 1 illustrates a structure of a pixel of a typical active matrix type organic light emitting diode (AMOLED) device according to the related art.
Referring to FIG. 1, a scanning line 1 is formed in the first direction, and signal and power supply lines 2 and 3 are formed in the second direction perpendicular to the first direction. The scanning line 1 and the signal and power supply lines 2 and 3 define a pixel region by crossing one another. A switching thin film transistor 4 functioning as an addressing element is formed at a portion adjacent to the intersection of the scanning line 1 and the signal line 2. A storage capacitor 6 is electrically connected to the switching thin film transistor 4 and the power supply line 3. A driving thin film transistor that functions as a current source element 5 is electrically connected to the storage capacitor 6 and the power supply line 3. An electro luminescent diode 7 is electrically connected to the driving thin film transistor 5. More specifically, the active matrix type organic light emitting diode (AMOLED) device has the switching thin film transistor 4 and the driving thin film transistor 5 in a pixel. The switching thin film transistor 4 is for addressing a pixel voltage, which is a gate driving voltage, and the driving thin film transistor 5 is for controlling a driving current of the active matrix type organic light emitting diode (AMOLED) device. In addition, the active matrix type organic light emitting diode (AMOLED) device requires the storage capacitor 6 to maintain a stable pixel voltage.
FIG. 2A illustrates a plane view of a thin film transistor for a typical inverted staggered type active matrix organic light emitting diode (AMOLED) device according to the related art.
Referring to FIG. 2A, a gate electrode 12 is formed in one direction, and a semiconductor layer 14 having an island pattern is formed to cover the gate electrode 12. Source and drain electrodes 16 and 18 are formed on the semiconductor layer 14 to respectively overlap portions of the gate electrode 12. The source electrode 16 is extended from the power supply line 20, which is formed in the same direction as the gate electrode 12. Although not shown in FIG. 2A, the drain electrode 18 is electrically connected to a first electrode (not shown) for the active matrix type organic light emitting diode (AMOLED) device, and the gate electrode 12 is electrically connected to a drain electrode (not shown) of the switching thin film transistor. Although not shown in FIG. 2A, the active matrix type organic light emitting diode (AMOLED) device has a storage capacitor (not shown) that is electrically connected to the power supply line and functions to stably maintain a pixel driving voltage for a period of time.
In the active matrix type organic light emitting diode (AMOLED) device according to the related art, the gate electrode 12 overlaps portions of the source and drain electrodes 16 and 18 keeping a spaced distance “d1” between the source and drain electrodes 16 and 18. A first overlapping portion “a” between the gate electrode 12 and the source electrode 16 is symmetrical with a second overlapping portion “b” between the gate electrode 12 and the drain electrode 18 with respect to the centerline of the gate electrode 12.
FIG. 2B illustrates a cross-sectional view taken along line IIB—IIB of FIG. 2A.
Referring to FIG. 2B, the gate electrode 12 is formed on a substrate 10, and a gate insulating layer 13 is formed on the entire substrate 10, on which the gate electrode 12 is already formed. The semiconductor layer 14 is formed on the gate insulating layer 13 to cover the gate electrode 12. The source and drain electrodes 16 and 18 are formed on the semiconductor layer 14 and spaced apart from each other. Parasitic capacitances Cgs and Cgd are generated in the first and second overlapping portions “a” and “b”, respectively. The parasitic capacitances Cgs and Cgd have a same capacitance value. In the active matrix type organic light emitting diode (AMOLED) device according to the related art, the areas of the first and second overlapping portions “a” and “b” are minimized to reduce the parasitic capacitances Cgs and Cgd. On the other hand, since a thin film transistor of the related art liquid crystal display (LCD) device only functions as a switching element and a data voltage is controlled depending on a common voltage, the switching thin film transistor generates a parasitic capacitance, and the parasitic capacitance induces a flicker phenomenon and a cross-talk phenomenon that deteriorate the quality of displayed images.
However, in the active matrix type organic light emitting diode (AMOLED) device, since a gray scale of an image must be displayed by controlling an amount of current in the driving thin film transistor, it is very important to keep a constant current to obtain a good quality of displayed images. More specifically, if the voltage cannot be stably maintained by the storage capacitor, a current level fluctuates greatly, and thus images having good picture quality cannot be displayed. Accordingly, the storage capacitor plays a very important role in the active matrix type organic light emitting diode (AMOLED) device. The active matrix type organic light emitting diode (AMOLED) device is a current modulation element, in which source and drain directions of a pixel voltage are already decided, and a polarity reversion from positive to negative, and vice versa, does not occur unlike in the liquid crystal display (LCD) device.
In spite of the above-described difference between the active matrix type organic light emitting diode (AMOLED) device and the liquid crystal display (LCD) device, an overlapping area between the gate electrode and the source electrode is formed to be the same as an overlapping area between the gate electrode and the drain electrode in the driving thin film transistor, such as in a switching thin film transistor. Thus, the amount of a pixel voltage depends on the size of an additional storage capacitor. Accordingly, a disconnection of the adjacent metal patterns may occur in a storage capacitor region, which then results in a product failure.